1. Field of the Invention
The present invention relates to an image reader for reading an image of a document with the use of an image sensor.
2. Description of the Prior Art v In such an image reader mentioned above, an image of a document is read by the image sensor and obtained image data is the digitalized and, further, digitalized data is binarized with the use of a predetermined threshold. Binary image data thus obtained is outputted to an external apparatus such as a printer.
In an ordinal character document wherein characters of black color are printed on a white background, the white background and black characters are clearly distinguished from each other.
FIG. 10 shows a frequency distribution of output data outputted from individual picture elements of the image sensor such as a CCD (charge coupled device) image sensor when a character document is read thereby. As shown clearly therein, peaks corresponding to black data and white data appear apart from each other. Accordingly, a fixed threshold is used to discriminate character data from the background data and a minor change in the value of the fixed threshold does not affect to a document image that is to be outputted.
However, if the document is a blue copy in which characters of blue color are formed on a blue background, respective peaks corresponding to characters and the background appear close to each other, as shown in FIG. 11.
In such a case, characters are erased or background data is overlapped over characters when a fixed threshold is used for binarizing image data.
Further, the method for varying the intensity of exposurement stepwise is not effective in such a case.